The study focused on hormonal therapies called aromatase inhibitors, which certain post-menopausal breast cancer patients take for up to five years after surgery. Aromatase inhibitors, or AIs, can reduce the risk of relapse by up to half, said co-author Jennifer Ligibel of Boston's Dana-Farber Cancer Institute.

Many women who take AIs experience joint pain or stiffness. In fact, joint pain is the leading reason that women stop taking the drugs early, Ligibel said. AIs also can cause other disruptive side effects, including hot flashes and brittle bones. As a result, up to half of women stop taking AIs or other hormonal therapies earlier than prescribed - a choice that can reduce their odds of survival.

A new study presented today at the San Antonio Breast Cancer Symposium looked at whether exercise could help relieve joint pain. Researchers randomly assigned half of participants to a supervised exercise program, involving 2.5 hours of aerobic exercises such as walking, as well as two strength training sessions overseen by a coach with experience working with breast cancer survivors.

After a year, women assigned to the exercise program had pain scores that were 20% lower than at the start of the study. Pain scores for women in the control group, who followed their normal routines, fell 3% in the same time, according to the study, led by Melinda Irwin, who leads the Yale HOPE (Hormone & Physical Exercise) Study.

Women who exercised more had better results.

Those who attended at least 80% of exercise sessions reported a 25% drop in their worst pain scores. Those who attended fewer than 80% of scheduled sessions had a 14% drop, according to the study of 121 postmenopausal women.

Women who improved on measures of heart and lung fitness by at least 5% also did better, with a 29% reduction in their worst pain scores. Women with a smaller increase in fitness saw only a 7% decrease in their worst pain scores, according to the study,

"No matter how you looked at joint pain, it got better with exercise," Ligibel said.

The study is one of the first to find a way to reduce joint pain in these women, Ligibel said.

Other studies have shown that exercise can help breast cancer survivors. It can prevent weight gain, which may reduce the risk of a relapse. Cardiovascular fitness is important for breast cancer survivors, who can suffer heart damage from certain chemotherapy drugs.

And a 2009 study in the New England Journal of Medicine found that women with lymphedema who followed a carefully designed strength-training program can help relieve lymphedema, a painful swelling in the arms that can occur after women have lymph nodes under their arms removed. Women in the program built muscle while cutting their risk of painful flare-ups in half.

Ligibel said it's important for breast cancer survivors to work with coaches who have received special training, to avoid increasing the risk of injury and lymphedema.

AIs work only in the 70% of breast cancer patients whose tumors are fueled by estrogen. The drugs lower estrogen levels indirectly, by preventing an enzyme called aromatase from converting other hormones into estrogen. That starves the tumors of the fuel they need to grow and spread. They also only work in women whose ovaries are no longer producing estrogen. Premenopausal women can get similar benefits from a drug called tamoxifen.

Ligibel said researchers can't explain why exercise helped these women. Earlier research has shown that exercise can improve mood and relieve symptoms of mild or moderate depression. It's also possible that exercise reduced inflammation, said Ligibel, who said her team measured markers of inflammation in the blood. Those results will be presented in a future paper.

Exercise doesn't help everyone.

San Francisco breast cancer survivor Eve Harris, 56, maintained a healthy weight throughout her breast cancer treatment by swimming, walking and practicing yoga. Yet she suffered such severe joint pain that she had to stop taking AIs after 18 months.

Finding a way to help women tolerate AIs is important, said Victoria Shanmugam, an associate professor at the Georgetown University School of Medicine.

Research suggests that "it is precisely those women who get joint complaints who should remain on these medications because they are the women who are likely to have the greatest benefit," Shanmugam said.